1. Field of the Invention
The present invention relates to a current position calculating device which is mounted in a mobile object such as a vehicle or the like and measures a travel distance, a travel direction, etc. of the mobile object to calculate a current position of the mobile object on the basis of the measurement result.
2. Description of Related Art
In a conventional current position calculating device for calculating the current position of a vehicle running on a road, the current position of the vehicle is generally calculated on the basis of a travel direction (azimuth) of the vehicle which is measured by an azimuth sensor such as a gyro, and a travel distance of the vehicle which is measured by a vehicle speed sensor or a distance sensor.
The travel distance of the vehicle is generally calculated by measuring the number of rotations of the output shaft of a transmission or a tire and then multiplying the measured number by a distance coefficient which corresponds to a distance at which the vehicle runs per one revolution of the tire.
Further, Japanese Laid-open Patent Application No. Sho-63-148115 discloses the following technique. That is, roads located within a predetermined region containing at the center thereof the current position of a vehicle, which is determined on the basis of a travel distance and an azimuthal variation (variation of the travel direction of the vehicle), is extracted from a road map. Thereafter, on the basis of the correlation between the presumed current position and each of the extracted roads, the current position is corrected so as to be located on a road which has the highest correlation with the presumed current position, thereby correcting an estimation error of the current position which is determined on the basis of the travel direction and the travel distance of the vehicle. In many cases, the correlation between the presumed current position and the road is presumed on the basis of the distance between the presumed current position and the road, or the difference between the travel azimuth (direction) of the vehicle and the azimuth of the road (extending direction of the road).
This technique is called as a map matching technique because the presumed current position of the vehicle is corrected so as to be matched to a road. According to the map matching technique as described above, the precision of the calculation of the current position can be enhanced.